1. Field of the Invention
The present invention relates to a toner and a developer using the toner for developing electrostatic images in electrophotography, electrostatic recording and electrostatic printing, and to an electrophotographic image forming method, an electrophotographic image forming apparatus and a process cartridge using the toner.
2. Discussion of the Background
Typically, a developer used in electrophotography, electrostatic recording and electrostatic printing and the like adheres to an image bearer such as a photoreceptor an electrostatic image is formed on in a developing process; transfers onto a transfer medium such as transfer sheets in a transfer process; and fixes on the transfer sheet in a fixing process. As the developer for developing the electrostatic image formed on the surface of the image bearer, a two-component developer including a carrier and a toner and a one-component developer (magnetic or nonmagnetic toner) not needing a carrier are known.
In two-component developing methods, toner particles adhere to the surface of the carrier and the developer deteriorates. In addition, a concentration of the toner in the developer decreases because only the toner is consumed, and a mixing ratio of the toner and the carrier has to be maintained at a specified ratio. Therefore, an image developer is comparatively large. To the contrary, in one-component developing methods, an image developer becomes smaller because of a sophisticated developing roller and the like.
Recently, office automation and colorization in offices are progressing, and opportunities in which not only copies of just letters but also a number of copies including graphs made by personal computers, images photographed by digital cameras, pictorial images, etc. are produced by printers for presentation increase. The produced image is mostly a complicated combination of solid images, line images and halftone images, and in accordance with this trend, a variety of market demands as well as demands for reliability of the image are increasing.
Conventionally, there is a magnetic one-component developing method using a magnetic toner and a non-magnetic one-component developing method using a non-magnetic toner in the electrophotographic process using the one-component developer. The magnetic one-component developing method is mostly used for compact printers recently, in which a developer bearer including a magnetic field generation means such as magnets bears a magnetic toner including a magnetic material such as a magnetite, and in which a layer-thickness regulation member forms a thin toner layer on the developer bearer for development. However, the magnetic material has a color, and mostly a black color and it is difficult to color the magnetic toner.
To the contrary, in the non-magnetic one-component developing method, a toner feed roller is pressed against a developer bearer to feed the toner onto the developer bearer which electrostatically bears the toner, and a layer-thickness regulation member forms a thin toner layer on the developer bearer for development. This method has an advantage of being usable for colorization because of not including a colored magnetic material, and is mostly used for compact full color printers recently, which are lightweight and low cost because of not using magnets in the developer bearers.
On the other hand, the two-component developing method uses a carrier as means of charging and transporting the toner, and the toner and carrier are transported to the developer bearer after they are sufficiently agitated and mixed in an image developer. Therefore, the toner can stably be charged and transported for a long time and the two-component developing method can easily be used for a high-speed image developer.
However, compared with the two-component developing method, the one-component developing method still has many points to be improved. In the one-component developing method, defective charge and transport of the toner due to long-time use and high-speed development tend to occur since the method does not have stable charge and transport means like the carrier. Namely, in the one-component developing method, a contact and friction charge time between the toner and friction charge members such as layer-thickness regulation members is so short that a toner having a low charge or a reverse charge tends to increase more than in the two-component developing method using a carrier.
Particularly in the non-magnetic one-component developing method in which ordinarily at least one toner transport member transports the toner (developer) and an electrostatic latent image formed on a latent-image bearer is developed by the transported toner, the thickness of the toner layer on the surface of the toner transport member has to be as thin as possible. This is same for the two-component developing method in which a carrier having quite a small diameter is used. In addition, particularly when a toner having high electric resistance is used as a one-component developer, the thickness of the toner layer has to be significantly thin because the toner has to be charged by the developing device. This is because when the toner layer is too thick, only the surface thereof is charged and the toner layer cannot be uniformly charged. Therefore, the toner needs to be charged quicker and to keep an appropriate charge quantity.
Accordingly, a charge controlling agent is conventionally included in a toner in order to stabilize the charge of the toner. The charge controlling agent controls and maintains the friction charge quantity of the toner. Specific examples of the negative charge controlling agents include monoazo dyes, salicylic acids, naphthoic acids, metallic salts and metal complex salts of dicarboxylic acids, diazo compounds, boric complex compounds, etc. Specific examples of the positive charge controlling agents include quaternary ammonium salt compounds, imidazole compounds, nigrosin, azine dyes, etc.
However, most of the charge controlling agents have colors and cannot be used for a color toner. In addition, some of the agents do not have good solubility with a binder resin and the agents on the surface of the toner, which largely affect the charge thereof, easily leave from the surface thereof. Therefore, the toner is unevenly charged, and toner filming over a developing sleeve and a photoreceptor tend to occur.
Accordingly, although images having good quality can be produced at the beginning, the image quality gradually changes and background fouling and image irregularity occur. Particularly, when the charge controlling agent is used for a toner for a full color copier producing continuous images while the toner is supplied to the copier, the charge quantity of the toner decreases and the color tone becomes noticeably different from that of the initial image. In addition, an image forming unit called as a process cartridge has to be changed quickly only after several thousand images are produced, which is a large environmental burden and gives troubles to users. Further, most of the units include heavy metals such as chrome and are becoming problems lately in view of safety.
Demands for printers are expanding lately, and down sizing, speeding up of printing and lowering cost of the printers are progressing. Accordingly, high reliability and long life of the printers are beginning to be required, and a toner capable of maintaining its properties for a long time is required as well. However, the above-mentioned resin charge controlling agents are unable to maintain the charge controllability, and contaminate a developing sleeve and a developer layer-thickness regulation members such as blades and rollers, resulting in lowering the chargeability of the toner and toner filming over a photoreceptor.
In addition, the developer is required to have a good chargeability because the development has to be performed by a small amount of a developer in a short time due to the downsizing and speeding up of printing. A variety of one-component and two-component developers are suggested, and a non-magnetic one-component developer is preferably used for a printer application because the printer can be smaller and lighter without using a carrier. In a developing method using the non-magnetic one-component developer, a toner is forcibly rubbed onto a developing roller or an amount of the toner thereon is regulated with a blade because of low toner feeding capability to the developing roller and toner retainability thereof. Consequently, toner filming over the developing roller tends to occur, resulting in shorter life of the developing roller, instability of charge quantity of the toner and poor development.
Further, a color toner for the non-magnetic one-component often includes a binder resin having a poor heat resistance and filming of the toner over the developing roller tends to occur.
Japanese Laid-Open Patent Publication No. 55-42752 discloses a salicylic acid or a metal complex thereof as a conventional charge controlling agent. However, a toner including the charge controlling agent does not have sufficient charge stability as desired and has poor resistance against environmental variation. Additives improve transportability, developability, transferability and preservability of the resultant toner besides controlling and maintaining a friction charge quantity thereof. Japanese Laid-Open Patent Publications Nos. 56-128956 and 59-52255 disclose a method of including a hydrophobic silica in a toner to improve these properties. However, the silica alone increases chargeability and transferability of the resultant toner too much, resulting in defective images such as scattered images and toner scattering.
Japanese Laid-Open Patent Publications Nos. 60-112052 and 4-40467 disclose a method of including titanium oxide or surface-treated titanium oxide with a coupling agent in a toner. However, the titanium oxide alone cannot impart a sufficient chargeability or a fluidity to the resultant toner. Further, the titanium oxide does not uniformly adhere to a toner because of its large particle diameter and secondary cohesion tendency, resulting in abnormal images.
Japanese Patent No. 3232858 discloses a surface-treated particulate titanium oxide. However, the surface-treated particulate titanium oxide alone cannot impart a sufficient charge quantity to the resultant toner yet although dispersibility of the surface-treated particulate titanium oxide is improved. Further, when the titanium oxide is used in combination with a silica, a charge quantity of the resultant toner adversely increases as time passes.
Further, Japanese Laid-Open Patent Publication No. 7-43930 and Japanese Patent No. 3160688 disclose a simultaneous use of a hydrophobic silica and hydrophobic titanium oxide. However, such a combination of additives having a high hydrophobicity increases a charge quantity of the resultant toner, resulting in defective transfer thereof.
As mentioned above, the conventional technologies have problems such as charge instability, instability against environmental variation, scattered images, toner scattering, insufficient fluidity, abnormal images due to the secondary cohesion, increase of charge quantity and defective transfer.
Because of these reasons, a need exists for a toner capable of stably controlling and keeping its charge quantity and chargeability; having less change due to environmental variation and good transportability, developability, transferability and preservability; and not adhering to a photoreceptor to produce abnormal images.